1. Field of the Invention
The invention relates to recombinant polypeptides and peptides, which can be used for the diagnosis of tuberculosis. The invention also relates to a process for preparing the above-said polypeptides and peptides, which are in a state of biological purity such that they can be used as part of the active principle in the preparation of vaccines against tuberculosis.
It also relates to nucleic acids coding for said polypeptides and peptides.
Furthermore, the invention relates to the in vitro diagnostic methods and kits using the above-said polypeptides and peptides and to the vaccines containing the above-said polypeptides and peptides as active principle against tuberculosis.
By “recombinant polypeptides or peptides” it is to be understood that it relates to any molecule having a polypeptidic chain liable to be produced by genetic engineering, through transcription and translation, of a corresponding DNA sequence under the control of appropriate regulation elements within an efficient cellular host. Consequently, the expression “recombinant polypeptides” such as is used herein does not exclude the possibility for the polypeptides to comprise other groups, such as glycosylated groups.
The term “recombinant” indeed involves the fact that the polypeptide has been produced by genetic engineering, particularly because it results from the expression in a cellular host of the corresponding nucleic acid sequences which have previously been introduced into the expression vector used in said host.
Nevertheless, it must be understood that this expression does not exclude the possibility for the polypeptide to be produced by a different process, for instance by classical chemical synthesis according to methods used in the protein synthesis or by proteolytic cleavage of larger molecules.
The expression “biologically pure” or “biological purity” means on the one hand a grade of purity such that the recombinant polypeptide can be used for the production of vaccinating compositions and on the other hand the absence of contaminants, more particularly of natural contaminants.
2. Description of the Prior Art
Tuberculosis remains a major disease in developing countries. The situation is dramatic in some countries, particularly where high incidence of tuberculosis among AIDS patients represents a new source of dissemination of the disease.
Tuberculosis is a chronic infectious disease in which cell-mediated immune mechanisms play an essential role both for protection against and control of the disease.
Despite BCG vaccination, and some effective drugs, tuberculosis remains a major global problem. Skin testing with tuberculin PPD (protein-purified derivative) largely used for screening of the disease is poorly specific, due to cross reactivity with other pathogenic or environmental saprophytic mycobacteria.
Moreover, tuberculin PPD when used in serological tests (ELISA) does not allow to discriminate between patients who have been vaccinated by BCG, or those who have been primo-infected, from those who are developing evolutive tuberculosis and for whom an early and rapid diagnosis would be necessary.
A protein with a molecular weight of 32-kDa has been purified (9) from zinc deficient Mycobacterium bovis BCG culture filtrate (8). This 32-kDa protein of M. bovis BCG has been purified from Sauton zinc deficient culture filtrate of M. bovis BCG using successively hydrophobic chromatography on Phenyl-Sepharose, ion exchange on DEAE-Sephacel and molecular sieving on Sephadex G-100. The final preparation has been found to be homogeneous as based on several analyses. This P32 protein is a constituent of BCG cells grown in normal conditions. It represents about 3% of the soluble fraction of a cellular extract, and appears as the major protein released in normal Sauton culture filtrate. This protein has been found to have a molecular weight of 32 000 by SDS-polyacrylamide gel electrophoresis and by molecular sieving.
The NH2-terminal amino acid sequence of the 32-kDa protein of M. bovis BCG (Phe-Ser-Arg-Pro-Gly-Leu (SEQ ID NO:49)) is identical to that reported for the MPB 59 protein purified from M. bovis BCG substrain Tokyo (34).
Purified P32 of M. bovis BCG has been tested by various cross immunoelectrophoresis techniques, and has been shown to belong to the antigen 85 complex in the reference system for BCG antigens. It has been more precisely identified as antigen 85A in the Closs reference system for BCG antigens (7).
Increased levels of immunoglobulin G antibodies towards the 32-kDa protein of M. bovis BCG could be detected in 70% of tuberculous patients (30).
Furthermore, the 32-kDa protein of M. bovis BCG induces specific lymphoproliferation and interferon-(IFN-γ) production in peripheral blood leucocytes from patients with active tuberculosis (12) and PPD-positive healthy subjects. Recent findings indicate that the amount of 32-kDa protein of M. bovis BCG-induced IFN-γ in BCG-sensitized mouse spleen cells is under probable H-2 control (13). Finally, the high affinity of mycobacteria for fibronectin is related to proteins of the BCG 85 antigen complex (1).
Matsuo et al. (17) recently cloned the gene encoding the antigen α, a major protein secreted by BCG (substrain Tokyo) and highly homologous to MPB 59 antigen in its NH2-terminal amino acid sequence, and even identical for its first 6 amino acids: Phe-Ser-Arg-Pro-Gly-Leu (SEQ ID NO:49).
This gene was cloned by using a nucleotide probe homologous to the N-terminal amino acid sequence of antigen α, purified from M. tuberculosis as described in Tasaka, H. et al., 1983. “Purification and antigenic specificity of alpha protein (Yoneda and Fukui) from Mycobacterium tuberculosis and Mycobacterium intracellulare. Hiroshima J. Med. Sci. 32, 1–8.
The presence of antigens of around 30–32-kDa, named antigen 85 complex, has been revealed from electrophoretic patterns of proteins originating from culture media of mycobacteria, such as Mycobacterium tuberculosis. By immunoblotting techniques, it has been shown that these antigens cross-react with rabbit sera raised against the 32-kDa protein of BCG (8).
A recent study reported on the preferential humoral response to a 30-kDa and 31-kDa antigen in lepromatous leprosy patients, and to a 32-kDa antigen in tuberculoid leprosy patients (24).
It has also been found that fibronectin (FN)-binding antigens are prominent components of short-term culture supernatants of Mycobacterium tuberculosis. In 3-day-old supernatants, a 30-kilodalton (kDa) protein was identified as the major (FN)-binding molecule. In 21-day-old supernatants, FN was bound to a double protein band of around 30 to 32-kDa, as well as to a group of antigens of larger molecular mass (57 to 60 kDa) (1).
In other experiments, recombinant plasmids containing DNA from Mycobacterium tuberculosis were transformed into Escherichia coli, and three colonies were selected by their reactivity with polyclonal antisera to M. tuberculosis. Each recombinant produced 35- and 53-kilodalton proteins (35K and 53K proteins, respectively) (“Expression of Proteins of Mycobacterium tuberculosis in Escherichia coli and Potential of Recombinant Genes and Proteins for Development of Diagnostic Reagents”, Mitchell L Cohen et al., Journal of Clinical Microbiology, July 1987, p. 1176–1180).
Concerning the various results known to date, the physico-chemical characteristics of the antigen P32 of Mycobacterium tuberculosis are not precise and, furthermore, insufficient to enable its unambiguous identifiability, as well as the characterization of its structural and functional elements.
Moreover, the pathogenicity and the potentially infectious property of M. tuberculosis has hampered research enabling to identify, purify and characterize the constituents as well as the secretion products of this bacteria.